The applicant herein is the applicant in co-pending U.S. application Ser. No. 07/253,703 filed Oct. 6, 1988, and assigned to a common assignee herewith--and corresponding to EPO Publication No. 0 311 460 published Apr. 12, 1989. That previous application teaches a battery charger where a principal feature is the fact that the battery charger can deliver a current to a rechargeable battery or cell initially at a rate in amperes greater than the capacity in ampere-hours of the battery--in other words, at a rate greater than 1C. Therefore, the rechargeable battery or cell being charged may be rapidly charged.
Another feature of the previous invention is that means are provided for detecting the internal resistance free voltage of the rechargeable battery or cell being charged, and comparing it to a pre-selected reference voltage which is independent of the battery being charged. In other words, for a particular type and rating of rechargeable battery or cell to be charged, a reference voltage is pre-selected and is generated within the charger circuits. (It is possible that the reference voltage may be pre-selected by switch setting or the like, with prior knowledge of the condition, rating, and type of rechargeable battery or cell to be charged.) The resistance free voltage is compared to the internally generated reference voltage at an instant in time when the charging current being delivered to the rechargeable battery or cell has been interrupted.
The prior invention provides that, as the internal resistance free voltage of the rechargeable battery or cell being charged exceeds the pre-selected reference voltage, means are provided to reduce the electrical charging current and thereby reduce the rate of charging the rechargeable battery or cell, in order to maintain the internal resistance free voltage at a value equivalent to the pre-selected reference voltage. In other words, if it is noted that the internal resistance free voltage of the rechargeable battery or cell being charged marginally exceeds the reference voltage, then that is an indication that the rate of charging current delivered to the rechargeable battery or cell is too high, and the rate of delivery of the charging power--i.e., the charging current--is reduced.
The present invention provides circuits that are in some way similar to those described in the above referenced U.S. application and EPO published specification, in that it has been determined that conditions may exist when it is desirable to have better control over the charging process, or conditions may exist where it is important to have control over the reference voltage against which the resistance free terminal voltage is being compared, so as to prevent unwanted overcharging characteristics of any sort. Overcharging may occur in some circumstances, for example in the event that the internal temperature of the rechargeable battery or cell is or becomes high, or even in the event that the ambient temperature in which the charger is operating is or becomes high. Further, it is sometimes important to monitor not only the resistance free terminal voltage of the rechargeable battery or cell being recharged, but also the rate of charge, because the onset of certain charge current conditions may be indicative of an unwanted overcharge condition occurring.
Recharging may occur in respect of a great many different kinds of rechargeable batteries or cells. Common conditions and types, however, particularly include nickel-cadmium that may be used in household toys and appliances; and more particularly for such rechargeable batteries and cells--especially nickel cadmium--which are used in significant quantities in products such as rechargeable hand tools and video camcorders. Other rechargeable batteries (or cells in some conditions) may be lead-acid systems: they may be found in very small sizes in portable audio tape/radio devices; and in much larger embodiments in forklift trucks, golf carts and the like, and electric vehicles. The voltage and capacity of such lead acid batteries may be from 2 volts (for a single cell) up to hundreds of volts and more, with capacities rated from fractions of an ampere-hour to thousands of ampere-hours. Obviously, particularly for large battery installations, it is desirable to provide charging currents consistent with the method of rapid charging, and if the rate of charging may be in the range of 5C to 10C or 15C, then the charging current may be in the range of several hundreds or thousands of amperes.
It must be noted that battery charging occurs when there is a capability of the rechargeable battery or cell to accept charging current--in other words, battery charging occurs as a function of the charging current and of the state of charge of the battery or cell being charged. In order for there to be a flow of current from the charging circuits to the rechargeable battery or cell to be charged, there is a terminal voltage for the charging circuit provided that is higher than the rest voltage of the cell or battery to be charged. There is, therefore, by the difference between those two voltages, a driving voltage--often referred to as "overvoltage" or "polarization"; and that voltage may, itself be controlled. But, it is also important to note that the cell voltage or battery voltage being spoken of is the resistance free terminal voltage thereof--that is, the terminal voltage of the rechargeable battery or cell being charged at a time during its charging sequence when flow of the charging current to the battery or cell has been interrupted. This eliminates all voltage losses due to resistances anywhere in the charging circuit or within the battery or cell being charged, and is therefore a true indication of the electrochemical voltage of the battery or cell. It is also to be noted, however, that the determination of the resistance free terminal voltage is taken rather soon after the flow of charging current has been interrupted, so as to preclude internal changes occurring within the battery due to time dependent electrochemical effects. Thus, it is the steady state resistance free terminal voltage that is important to be detected. Those voltages differ, of course, for various kinds of cell or battery types: such as, for example, nickel cadmium (where the resistance free terminal voltage of a freshly charged cell may be in the order of about 1.38 volts, and of a substantially discharged cell at about 1.19 volts, about where the voltage for the most part remains at about 1.2 volts); or lead acid (where the resistance free terminal voltage may vary from about 1.90 volts to about 2.45 volts).
It is one of the purposes of the present invention, as described in greater detail hereafter, to assure that any temperature rise within a battery or cell being charged comes as a consequence solely of the thermodynamics of the charging reactions and of the internal resistance of the battery or cell, and not as a consequence of the overcharge electrochemical processes occurring in the cell. As a consequence, it is a corollary of that purpose that battery chargers in keeping with the present invention provide higher efficiency when compared with conventional battery chargers.
To achieve that purpose, the charging circuits must be capable of determining that point during the charging cycle when overcharging of the battery or cell is about to occur. In other words, the battery charger must be capable of the determining the instantaneous capability of the battery or cell to accept charging current, and to adjust the rate of delivery of the charging current accordingly. It happens that, by being able to demonstrate those characteristics, battery chargers according to the present invention have the effect of removing or eliminating the memory characteristic that is so prevalent with nickel cadmium batteries and cells--especially when the nickel cadmium battery or cell has been charged at a slow rate if it has not yet been fully discharged. It has been the practice, in the past, especially for persons using hand-held tools or camcorders, and the like, either to continue to operate the device until such time as it fails due to substantially complete discharge of the battery, or sometimes such as at the end of the day to remove the battery from the device and forcibly discharge it so as to assure that it has been fully discharged, before recharging it.
Moreover, when batteries and cells such as nickel cadmium are charged at a relatively low rate, it is possible that short circuits can occur within the battery, and that is much less likely to happen when the battery is charged at a high charging rate. Of course, in nearly every instance, battery chargers according to the present invention provide an initial high charging current if the battery or cell to be charged can accept such a current. As a consequence, it has been found that the cell life--that is, the number of recharge cycles to which a battery or cell may be subjected--may be increased by a factor of two or three in the case of nickel cadmium batteries or cells when they are consistently charged using battery chargers of the present invention.
Thus, battery chargers of present invention are capable of providing just small quantities of recharging energy to partially discharged batteries or cells, without harming them. That, in turn, suggest that designers of devices using such batteries can ultimately design them to use batteries having lesser capacity than at present, thereby resulting in those applications having lower capital cost of manufacture and of acquisitions by the user. By being able to provide a "topping up" charge to such as lead-acid batteries or cells, deep discharge and therefore the adverse effects of deep discharge on battery life, is avoided. Still further, because the present invention provides battery charges that are capable of recharging batteries in a very short period of time, the necessity for duplicate or standby batteries, or the necessity for taking the battery operated device out of service for a significant period of time to recharge the battery, are eliminated or overcome.
A typical example of the above might be a golf cart. Usually, golf carts have six 6-volt batteries each having a capacity of the about 134 ampere-hours. Such batteries have costs in the range of about $400.00,and the total weight is in the range of about 200 kilograms. If it were accepted that when the player using the golf cart returns the cart to be re-used by the next player, and that the next player will not use the cart for about 15 or 20 minutes, it is possible to provide the cart with three 12-volt batteries, each having a capacity of about 70 ampere-hours. That installation is capable of being recharged in about 15 or 20 minutes by battery chargers of the present invention; and such a battery installation may be obtained at a cost of approximately $200.00 and may have a weight of about 100 kilograms. Still further, a lighter golf cart can, itself, be designed, so that its range may be extended or in any event its capital cost reduced due to the lighter battery weight that it might carry.
Another typical example is cordless--that is, hand-held --battery powered hand tools. It has been noted that manufacturers of such tools continually increase the size of the battery packs they require in order to provide them with longer operating periods; and that by providing heavier and larger battery packs, the tool becomes bulkier and heavier. Since it was the intention of battery powered hand-held power tools to be small and easy to handle and manipulate, the provision of heavier and bulkier battery packs is contrary to the initial purpose for which those tools were developed. On the other hand, by providing battery chargers in keeping with the present invention, the designer or manufacturer of the hand-held battery powered tools can bring to the public a tool with a much smaller battery. and which is therefore much easier to handle. The battery packs can be very quickly recharged, such as during a work break for refreshment, so that the capital cost of acquisition especially by professional tradesmen and the like can be reduced and convenience of use enhanced.
Still other circumstances may be such as for hand-held portable telephones or portable dictating machines, such as the one on which the present application has been drafted. Such machines--and portable audio machines in general, especially those having recording capabilities--may have various current demands placed on the batteries which power them, depending on whether they are in a recording or playback mode, or if they are rapidly spooling tape from one reel to the other in the machine.
It should also be noted that in a further alternative embodiment of the invention, as discussed hereafter, means are provided for determining the internal pressure of the battery or cell being charged, and to alter or terminate the charging operation as a consequence of the sensed internal pressure.